How is DGA (Dissolved Gas Analysis) related to health of transformer?

Transformer oil is the ‘blood’ of transformer.

It bathes every component of transformer and contains lot of diagnostic information to evaluate (under Dissolved Gas Analysis) the health of transformer.

What are ‘fault gases’?

‘Fault Gases’ are formed in transformer oil due to natural ageing and as result of faults inside transformer.

Gas formation includes oxidation, vaporization, insulation decomposition, oil breakdown and electrolytic action.

The gases of interest are Hydrogen, Methane, Acetylene, Ethylene, Ethane, Carbonmonoxide and Carbon-di-oxide.

What are the important steps of DGA?

The three important steps are,

Sampling of Transformer Oil using Oil sampling tube, without any lose to its gas contents.

Complete extraction of fault gases using Multi Stage Extractor with efficiency of more than 98%, since the results are interpretation of concentration
     of fault gases in sample oil.

Analysis of fault gases for its contents and quantity in ppm(parts per million) using Gas Chromatograph

How are the DGA results interpreted and why does complete extraction important?

Fault gases are completely extracted from a known volume of transformer oil and the volume of fault gases are measured at STP.

The Total Gas Content (TGC) of fault gases in sample oil is observed in ‘millilitre (ml) of gases per millilitre (ml) of oil’

Then converted into ‘microlitre of gases per litre of oil

OR

Parts per million (ppm)

Then the quantity of individual fault gases is measured in ppm using a Gas Chromatograph.

Hence to get accurate results ‘complete extraction’ is very much needed.

What are the different types of DGA?

Vacuum Extraction Method (ASTM D-3612- 02)

Sample Oil is exposed to vacuum and evolved gases are compressed using mercury. This method has two types depending on its extraction efficiency.

Single Extraction method: Oil sample is exposed to simple vacuum, once only and the evolved gases are compressed for volume measurement.

This type has an extraction efficiency of about 45% only and hence obsolete now.

Multistage Extraction method: In this latest novel method, the same sample oil could be subjected to high vacuum, repeatedly thrice or more.

And the efficiency achieved is more than 98% in three extractions itself.

This patented Multistage Extraction method reveals all the dissolved gases in their actual concentrations.

D-TOMGET, Model:1207 is designed for Multistage Extraction.

D- TOMGET, Model:1207 is simple, very accurate and cost effective.

The Stripping Method:

Nitrogen is allowed to flow through the sample oil as a carrier gas. Efficiency of the method is very much limited and the system is more complicated.

Head space analysis:

Argon is used as blanket in a vial of sample oil and agitated. Accuracy of results depends on various factors like precise volume of oil and blanket gas,

apart from temperature and pressure.

What are the important specifications followed in D-TOMGET and insisted by IS:9434:1992, ASTM D -3612 – 02 and CPRI ?

D-TOMGET, if subjected to a vacuum of 0.1 mbar and left, the vacuum remains unchanged.

Gas Expansion Chamber is ten times the volume of sample oil for complete extraction.

Extraction is done at a very high vacuum of 0.02 mbar

Sample oil can be exposed to vacuum many times, until complete extraction of fault gases is achieved.

Measurement of extracted gases shall be possible to the nearest 0.05 ml at atmospheric pressure.